1. Field of the Invention
The present invention relates to an integrated device. More particularly, the present invention relates to an alignment mark configuration.
2. Description of the Related Art
Photolithography is a critical step in the fabrication of an integrated device. For any general type of device fabrication, depending on the complexity of the product, the number of photolithography processes that are required to complete the product vary in number from about 10 to 18. In order to properly transfer a pattern on a mask to the wafer, the location of the alignment mark must be identified to accurately register the pattern on the mask with the previously formed pattern before performing the exposure process. The issue of discarding the entire wafer due to a mistake such as pattern misalignment in the pattern transferring process is thus prevented.
Proper identification of the alignment mark location mainly relies on the topography of the wafer surface. A scattering site or diffraction edge of an incident light is formed during the alignment process. The diffraction light reflected from the alignment mark is used as a signal, which is received by the overlay detector, to align the wafer and the mask.
The alignment mark is usually located on the scribe line and is formed concurrently with the shallow trench isolation structure. Since the last step in shallow trench formation is chemical mechanical polishing, damage may be inflicted upon the alignment mark to various extents due to the influence of the topography surrounding the alignment mark. If the damage inflicted on the alignment marks is too serious, the overlay detector cannot detect a clear alignment signal, causing a misalignment.
In general, the scribe line, depending on its orientation, is differentiated into the X-direction and the Y-direction. Surrounding the alignment mark in the Y-direction is usually a large flat region, whereas surrounding the alignment mark in the X-direction is often a large region of the trench structure. When a stepper and a KLA overlay machine are used to detect the extent of alignment between an upper layer and a lower layer, the overlay misalignment values detected in the X-direction and in the Y-direction are selectively presented in Table 1.
TABLE 1StepperKLA Overlay MachineX(3 sigma)Y(3 sigma)X maxY max1.5260.0140.7070.0441.5720.0181.3300.1051.3920.0151.0850.0500.8420.0240.7440.051
Based on Table 1, the extent of misalignment using the alignment mark in the X-direction for alignment is significantly greater than when using the alignment mark in the Y-direction.